A HISTORY OF SCIENCE 



drawn while the earth was regarded as the fixed 

 centre of the universe. 



In the eleventh century another Arabian discoverer, 

 Arzachel, observing the sun to be less advanced than 

 Albategnius had found it, inferred incorrectly that the 

 sun had receded in the mean time. The modern ex- 

 planation of this observation is that the measurement 

 of Albategnius was somewhat in error, since we know 

 that the sun's motion is steadily progressive. Arza- 

 chel, however, accepting the measurement of his prede- 

 cessor, drew the false inference of an oscillatory motion 

 of the stars, the idea of the motion of the solar system 

 not being permissible. This assumed phenomenon, 

 which really has no existence in point of fact, was 

 named the ''trepidation of the fixed stars," and was 

 for centuries accepted as an actual phenomenon. 

 Arzachel explained this supposed phenomenon by as- 

 suming that the equinoctial points, or the points of in- 

 tersection of the equator and the ecliptic, revolve in 

 circles of eight degrees' radius. The first points of 

 Aries and Libra were supposed to describe the cir- 

 cumference of these circles in about eight hundred 

 years. All of which illustrates how a difficult and false 

 explanation may take the place of a simple and 

 correct one. The observations of later generations 

 have shown conclusively that the sun's shift of position 

 is regularly progressive, hence that there is no "trep- 

 idation" of the stars and no revolution of the equi- 

 noctial points. 



If the Arabs were wrong as regards this supposed 

 motion of the fixed stars, they made at least one 

 correct observation as to the inequality of motion of 



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